mfd: rave-sp: Initialize flow control and parity of the port

Relying on serial port defaults for flow control and parity can result
in complete breakdown of communication with RAVE SP on some platforms
where defaults are not what we need them to be. One such case is
VF610-base ZII SPU3 board (not supported upstream). To avoid this
problem in the future, add code to explicitly configure both.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>